How to Install EPDM rubber roof

How to install a rubber roof

Firstly lets deal with the main problem with a rubber roof, their looks. They can and do sometimes look horrible. Jobbing builders, poor roofers and over stretched DIY fitters are normally the main culprits here. However if you plan your roof and give it just a little thought first, then prepare carefully and spend a little bit extra on some nice EPDM roof trims, not only will you have a superb hard wearing new roof but one that will actually look nice too.

This is what you don’t want…

How not to fit EPDM – I don’t know about you, but looking at this every day would upset me. Not only is it wrong, it’s plain ugly. Good rubber roof materials are also not cheap so why not fit them with some planning and make a nice one?

How to install an EPDM rubber roof

How to fit a rubber roof – The devil is in the detail, this separates a nice roof from a horrible one. A few extra pennies on some click together trims, a little bit of planning, will result in a roof that will last you up to 50 years. I’ll take you through the steps from start to finish as always.

In this article I will be picking the best videos made by rubber roof suppliers so that you can read my advice and see what they suggest also. This is purely because they have already made very clear good quality video instructions, and for me to remake their videos would be silly. What I will be adding is my 30+ years of flat roofing knowledge, put them in order and add things either not covered, or make suggestions I think will help you fit them in a professional manner.

Roof deck timber choice for EPDM

If you are fitting a one piece EPDM roof then you will need to glue it down, unless it’s ballasted or mechanically fixed (more on this later). The trouble with this is that the glues used in rubber roofs do not like the presence of contaminants such as bitumen or asphalt. For good adhesion your roof decking will need to be clean Plywood or OSB sterling board, and that will almost certainly mean a brand new roof deck.

OSB 3 – (Oriented Strand Board) These days this is fast becoming the roofers first choice. Engineered to specific tolerances, designed for the building trade, load bearing, and resistant to moisture. Allow 3mm (1/8“)gaps between all edges of the board.

OSB 3 T & G – (tongue and groove) This is great to work with, not only self supporting thanks to the T&G, but comes in the smaller 2.4m x 600 (8′ x 2′) size so it’s easier to lift and handle. Sourcing it can be a problem in some towns. See local stockist link below.

Plywood – Marine ply or a good quality exterior grade, make sure the plywood does not have knot holes missing from the face or large splinters and lumps. This will not only make the surface look ugly but may also puncture the membrane. Again 3mm gaps where the boards abut.

Full replacement or over boarding of a roof deck

Unless the flat roof is a new build you will be replacing an existing flat roof, normally a felt roof such as built up bitumen or Asphalt. As described earlier these substances are a contaminate to the glues used in EPDM and the membrane itself with direct long term contact. This will leave you with two choices…

Complete roof deck replacement – Ripping all previous traces of the old roof including felt / asphalt and contaminated timber to reveal roof joists, then renewing.

Over boarding – The fitting of new 18mm OSB 3 sterling board on top of the existing roof, or stripping the old roof covering and fixing directly over the previous timber roof deck.

When to consider over boarding a flat roof and when to replace

In my opinion it’s always nicer to fully strip a roof, however, there are some instances where this will be either so difficult that it increases the price of the job in terms of labour significantly, or the decking underneath is for instance marine plywood and has been nailed down with 6000 – 6″ inch nails. In these instances it may be sensible to consider your options rather than completely ruin any ceilings underneath.

Boarding on top – The roof structure and decking must be in good condition, rot free, and preferably not chipboard. Always strip down the roof back to the timber deck for a close visual inspection if possible. The benefit of over boarding is that any ceilings underneath will not be stressed by levering off the old timber deck.

Stripping a flat roof – With old flat roofs this can be most of the time, especially with chipboard. Often the chipboard has turned to ‘ Weetabix’, a crumbled mess that happens when chipboard is exposed to moisture for prolonged periods. If you must over board chipboard, always fix into the joists underneath.

OSB and Ply gaps – Leave a 3mm gap between all edges of any 8 x 4 board If the roof is very large or unusually long, check that you don’t need expansion breaks as well.

When a roof is abutting a wall at the perimeter of the roof, allow 2mm per meter for the distance that the roof protrudes from the wall, for expansion purposes. (Xmm gap in diagram above)

One piece rubber roof – fully adhered

This is the type of system I like to install best, one of the key strengths of a rubber roof is the lack of joints and therefore potential leaks, EPDM rubber membrane can be cut to virtually any size from one piece without joins up to 15m x 30m. (49 feet x 98 feet) Typically it’s a Firestone type branded rubber membrane in 1.2mm domestic grade or 1.5mm commercial thickness. The least joins you can have in the roof the better, and if you get caught by a shower whilst fitting, simply spread the rubber over the roof to keep it dry.
This video shows you the basic steps involved in a fully adhered EPDM roof, sometimes known as the classic bond.

Fitting EPDM membrane

Firstly make sure that the roof decking is 100% clean of dust and particles that will show up under the membrane or it can stop the glue bonding properly. Not only do we want it stuck down without contaminants, but we don’t want any goose eggs (bumps) underneath to spoil the aesthetics either. Use a soft brush, then get down on your hands and knees to have a look across the surface of the roof, then brush it again.

Now that your nice new timber roof decking is on and clean, it’s time to place the rubber membrane onto your roof. If you have read any other guides on how to fit a rubber roof you will have read that they advise you to allow the membrane time to relax for 20 minutes or so. This is so that the rubber is allowed time to contract or expand back to its neutral state, where it’s neither stretched or compressed by being packaged and shipped.

Relaxing time – This is very important advice, if you don’t want the roof to look like a wrinkled mess that ponds (holds water) and could be stretched away from the upstands later on. It also fits in very nicely with a tea and biscuit break, bonus.

I would also advise if possible you take this a stage further. Often when the EPDM membrane arrives it will be packed tightly into a bundle, often resembling some sort of world record attempt to fold it as many times as the laws of physics will allow. When my membrane arrives I unfold it fully on my lawn or in the garage (being very careful not to get it dirty), and roll it gently into a loose roll, this way when you come to use it on the day, any creases, folds and stresses in the membrane have already disappeared. Then allow the 20 min’s of relaxing as advised and you will have a roof so flat you’ll be playing snooker on it later.

Cutting EPDM membrane

Final trimming is usually done near the end of the job with a sharp Stanley knife, but often you will have to trim off excess membrane if it’s too big to work with. Always think twice before you make any cut, I like to use sharp scissors because you have less chance of accidentally running off course with them, as can happen with sharp knife blades and enthusiasm.

Gluing down the EPDM rubber

This is normally done in two stages… Always read the suppliers or manufacturers instructions on the sides of the containers, mix thoroughly when told to do so, and observe temperature guidelines.

Stage 1 – Water based adhesive (WBA) – After gently folding back one half of your membrane, the WBA is applied to the roof decking in a nice even coating with a roller, avoid any lumps or dry spots as this can give the roof a cellulite appearance. Keep the glue back 150mm (6″) from the edges of the roof in all directions as your contact adhesive will be here later on.

Some people recommend coating the membrane and roof deck with a thin coat of adhesive, but I’m not sure it makes a difference either way, as long as you don’t allow the glue to dry. Now roll out the membrane from the middle outwards to avoid wrinkles, then bush the membrane onto the glue with a soft bristle brush, firm but gentle, you don’t want to introduce stretching but you need the put the rubber membrane in full contact with the glue. Now do the same for the other half of the roof.

The majority of your roof is now glued down. This water based adhesive will dry slower than the contact adhesive you will be using next, so while it’s perfectly fine to walk on it, try not to drag your feet, kit or heavy items across it, as it may stretch or move the membrane.

Stage 2 – Contact adhesive – This is for roof details and edges. This is a much stronger adhesive, not only will it allow you to stick the membrane into corners, up stands and other tricky details, but the 150mm around the edge of the roof helps to stop accidental movement or wind lift when working on a windy day.

Always apply the contact adhesive to both surfaces and allow it to become touch dry before placing the membrane in contact with the substrate. Use a silicone roller to roll the membrane down and a penny roller if necessary on the top surface when glued.

Stage 3 Optional – The Russ strip (Reinforced Universal Securement Strip) This is often overlooked but EPDM over time may shrink somewhere between 1 – 10%. This can cause details like upstand edges or upstand walls to shrink and be pulled away on a large roof over the years.

If you have a large roof, or a roof with lots of details like parapet walls or upstand flashing, you may want the consider investing in Russ strips and mechanically fixing every 300mm (12″) at the perimeters where a change of angle occurs. When the edge details like up stands are fully secured in this manner, the expansion of the membrane should allow for any other movement. Always ask your supplier if you are unsure.

EPDM details

The details normally come next, and what I mean by details are any internal, or external corners, up stands, EPDM joins, flashings, outlets, pipes through the roof or anything else that is not a simple matter of just laying a big square flap of membrane onto the roof. See video instructions of fitting EDPM

EPDM roof trims

This is where people go wrong a lot in my opinion. Shop around to find the trim finish that will look the best to suit your application. The cheapest option is usually the metal edge trim and it can come with a metal gutter edge trim also, and there’s absolutely nothing wrong with it from a practical perspective. But when used in conjunction with for instance, galvanised hex bolts and plastic caps it can make a domestic roof look seriously ugly, especially if the bolts are visible from the floor or a nearby window.

Consider some plastic roof trims for small domestic roofs, compare a couple of suppliers and see what sort of edging trims and gutter drip systems they have to offer. There are some really nice profiles on the market these days in both black and white that will make all the difference to the final finished roof. One example of such a product is the excellent two part nail less ‘ Permaroof ‘ gutter edge trim which makes gutter drip trims look really neat.

EPDM Edge trims

Around the perimeter of a roof that is not a run off point for water, (drip) you will need to keep the water on the roof and drive it towards your drip trim. This is done with kerb edge trims, these not only keep the water on the roof thanks to a built in up stand, but also cloak the fascia to make the finish look smart from the floor.

Kerb edge trims – The plastic versions sit on either a bead of mastic (sold by manufacturers) or foam tape which is compressed between the underneath of the trim and the top of the EPDM to form a water tight barrier. When the trim is in place it’s simply fixed with stainless steel plastic topped pins.

Metal edge trims – These can look fine depending on the type and style of fixings, and whether it can be overlooked from above. If it’s for an extension roof that can be seen every day from bedroom windows you may want to think again. Also any Kerbed edges like the ones pictured will have to be created with timber underneath the membrane first.

Plastic edge trims – Typically they come in black or white, and in a few different styles of profile. A whole range of joints, corners and other details like end caps are also available. Always remember to allow expansion where two lengths of trim are jointed, typically this is about 5mm but check with your supplier.

EPDM Drip trims

This is the other main type of roof trim, all roofs require a water run off point that drips into a gutter, unless you have a outlet or pipe in the roof instead. If you’re using metal trims, the metal drip trim can sometimes double up as a wall trim for flashing into the brickwork, and these would usually be used when the rest of the roof trims are also metal. If you’re using plastic roof trims there are two popular types of plastic drip trims shown below…

The two part drip trim – Consisting of a back plate that is nailed flush with the top of the roof deck. The EPDM membrane is then sandwiched between this, a bead of mastic, and the front trim, which is finally nailed with stainless steel plastic topped pins to finish.

Two part clip together – This type of drip trim comes with a back plate which is nailed to the roof where you require the drip. The other section clips in without nails, securely trapping the membrane without the need for additional nailing. Also sports a ‘Kick out’ to drip the rain water into the gutter.

EPDM Flashings

The last stage of fitting a new EPDM roof is normally fitting the over flashings where a roof abuts a brickwork wall. This is normally done with the plastic coated metal flashing strip called a wall trim but can also be the metal gutter drip trim as they are sometimes universal. These are chased into the mortar between the brickwork, usually at a height of around 150mm (6″) from the roof deck.

Alternatively lead flashings can be used but they do not allow for face fixings that clamp the rubber membrane to the brickwork upstand in the same fashion if this is required. See how to fit lead roof flashing here.